Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of integrated circuits. Its most commercially important implementations include a magnetron positioned in back of the target being sputtered. As originally developed for the semiconductor industry, sputtering deposits aluminum and other conductive materials onto the surface of silicon wafers, in the surface of which there may be formed holes or other features having relatively mild aspect ratios. Many of the magnetron sputter reactors developed for this use and still being used include a magnetron having inner and outer poles of equal intensity in a nested structure having a substantially constant annular gap between the poles with an encompassing size of the magnetron that is only somewhat smaller than the target being sputtered. The magnetron is nonetheless rotated about the target center to scan and uniformly sputter the entire target. The primary design consideration in this design is uniform erosion of the target although some consideration is given to the fact that the uniform deposition on the wafer from a target only somewhat bigger than the wafer is not satisfied by uniform target erosion.
More recently, sputtering has been increasingly applied to deposit thin conformal layers into holes formed in dielectric and other layers having a high aspect ratio, that is, a large value of the ratio of the depth to the width of the hole. An example of a high-aspect ratio hole is via hole formed through a dielectric layer to provide a vertical interconnect. Such a hole may have a depth of 1 μm and a width of less than 100 nm. Another example is a narrow trench for a memory capacitor. It has been found that sputtering into such unfavorable geometries can be accomplished with a small but strong magnetron that is at least partially unbalanced, that is, its outer pole has a total magnetic intensity, which is the magnetic flux integrated over the outer pole, that is greater than the total magnetic intensity of the inner pole. The unbalance may vary around the annular gap between the two poles. Uniform erosion and deposition with such a small magnetron is achieved by rotating it about the target center, but the small magnetron in this design approach should still extend towards the target center.
Sidewall coverage is typically the most crucial aspect of sputtering thin conformal layers into high aspect-ratio holes. The sidewall thickness must be sufficient to provide the required barrier or seed function but not so thick as to block the hole. Further, the sidewall coverage must be fairly uniform along the sidewall height since any point having insufficient thickness, particularly for a barrier layer, introduces a defect in the entire chip. As a result, deposition uniformity over the radius of the wafer is not considered as the most crucial issue. Instead, sidewall uniformity and sidewall asymmetry between the radially inner and outer sidewalls have become the crucial issues.
It has been found that sidewall asymmetry is reduced when a small magnetron preferentially and perhaps solely sputters the periphery or edge of the target and does not sputter the central portion of the target. Further, a large number of the sputtered atoms are ionized, and strong wafer biasing draws the ions into the holes at near vertical angles. Such edge sputtering, however, introduces a further problem. Sputter deposition involves transporting material sputtered from a target to the wafer. The transport may involve some scattering from the sputtering gas or other residual particles such that some of the sputtered atoms are redeposited on the target. Also, ionized sputtered atoms may be attracted back to another area of the target. Sputtered material redeposited in an area of the target being substantially sputtered presents little problem since it is immediately resputtered. That is, even with redeposition, on net the target is being sputtered and eroded in that area. However, if the sputtered material is redeposited in an area of the target not being substantially sputtered, for example, the target center during edge sputtering, the redeposited material at the center on net is not resputtered but instead contributes to a growing thickness of the redeposited material on the target. Redeposited target material does not bind well with the original target material, particularly when its thickness grows to tens or hundreds of microns. The problem is intensified when reactive sputtering is being used to deposit a reacted layer, for example, the barrier material tantalum nitride by sputtering tantalum in the presence of nitrogen within the sputtering chamber. Thicker nitride layers when redeposited on the metal target do not stick well to it. In any case, the redeposited material is likely to flake from the target and introduce severe particulate problems when the flaking material falls on the wafer.
Redeposition, whether on the edge or at the center, can be circumvented by a careful design of the magnetron to assure that there positive sputtering occurs in all areas of the target, that is, redeposition occurs nowhere. However, this requirement reduces the design freedom for optimizing other sputtering characteristics, such as ionization fraction and deposition profile and uniformity. It is, instead, preferred that the redeposition problem be resolved by means other than the magnetron used in sputter deposition.
Rosenstein et al. in U.S. Pat. No. 6,228,236 address a different but related problem in which the outer periphery of the target is being preferentially redeposited. Rosenstein proposes a centrifugal mechanism rotatably supporting a single magnetron to additionally move it from an inner position to an outer position dependent upon the direction of rotation of the magnetron. At the inner position, the target is being used as a sputter deposition source for depositing target material onto a wafer; at the outer position, the target is being sputtered and cleaned with no wafer present in the chamber. Hong et al. in provisional application 60/555,992, filed Mar. 24, 2004, more directly address redeposition at the target center in which another centrifugal mechanism moves a magnetron from the outer deposition position to an inner cleaning position dependent upon the speed of rotation. Other mechanisms have been suggested for mechanically moving a magnetron in the radial direction while it is also rotating with respect to a target center. These mechanical solutions are effective but are considered less than ideal because of the added expense of the mechanical mechanism required for the radial movement and the reliability issues always present with moving parts, particularly ones which are immersed in a cooling bath and which need to quickly move mechanical members between different modes of operation.
Accordingly, it is desired to clean unused portions of a target without use of mechanisms that radially move the magnetron.
Gopalraja et al. have disclosed in U.S. Pat. No. 6,451,177 bimodal plasma states in a target formed with an annular vault and the magnetron includes several different types of magnets at different portions of the vault primarily affecting the ionization fraction.